Supreme Court observers uttered nary a peep on June 13, 2005 when the Court handed down its unanimous decision in Grable & Sons Metal Prods., Inc. v. Darue Eng’g & Mfg. But Grable—the Supreme Court’s first decision on the boundaries of substantial federal question (SFQ) jurisdiction since the 1986 case of Merrell Dow Pharmaceuticals, Inc. v. Thompson—opened the door to the federal courthouse for claims that Merrell Dow implied was shut (or only cracked open). Grable should revitalize the debate on the proper scope of SFQ jurisdiction, a species of federal-question jurisdiction applicable in certain cases in which the plaintiff has not affirmatively alleged a federal cause of action, but nonetheless seeks relief that requires the resolution ofsubstantial, disputed questions of federal law....